# Direct & Indirect Measurement of Ocean Currents

Instructor: Natalie Boyd

Natalie is a teacher and holds an MA in English Education and is in progress on her PhD in psychology.

The way that surface currents move is a major source of study in the field of oceanography. In this lesson, we'll take a look at both direct and indirect methods for measuring surface currents, including examples of each.

## Surface Currents

Alma lives by the ocean. She's noticed that the water moves constantly and she wonders how she could measure the movement of the sea. Alma is observing currents, which are movements of water. Specifically, the currents Alma can see are surface currents, which are movements at the shallowest parts of the water.

Surface currents are often guided by the wind blowing across the ocean. This is in direct contrast to deep water currents, which are caused by other things like temperature or salinity. Because they are caused by different things, surface currents and deep water currents also must be measured differently. To help Alma understand how she can measure the movement she's observing, let's take a look at the two main ways of measuring surface currents: direct and indirect methods.

## Types of Measurement

When Alma sits down to dinner, there are a couple different ways that she can know whether the food is too hot to eat or not. She could measure the temperature of the food with a thermometer, for example. Alternatively, if her husband says, ''Ouch! That burned my mouth,'' she could assume that the food needs to cool off. In one case, Alma is directly measuring whether the food is too hot by using a food thermometer. In the other case, she's not really measuring the food, but instead looking at her husband's reaction.

Measuring ocean currents is kind of like seeing if food is too hot to eat or not. Alma can use direct methods, which involve looking at the actual movement of the water. That would be like checking the food by using a thermometer. In contrast, Alma could also measure currents by using indirect methods, which involve looking at something other than the movement of the water and working backwards to figure out the current. That would be like using her husband's reaction to measure the heat of the food: she's not directly measuring the food, but instead indirectly measuring it by looking at his reaction.

## Direct Methods

Alma understands that direct and indirect methods of measuring surface currents are different. She even understands the basics of how they differ. But, what exactly are the ways of measuring these currents? To understand that, let's first take a look at the two most common direct methods of measuring surface currents: floating and fixed devices.

Floating devices are just what they sound like: they float in the water and measure surface currents by the rate and direction the devices move. Floating devices can be intentional (such as floats or trackers), which means that they are put in the water by scientists to measure the currents, or they can be unintentional (such as trash), which means that they happen to be in the water and can be used to measure currents.

For example, if Alma spots a pair of red sneakers in the water near her home and then several days later, those same sneakers are spotted a hundred miles south of her home, the surface currents could be calculated even though the sneakers were not intended to measure currents.

Since floating devices are used to directly measure surface currents by floating in the water, Alma is not surprised to learn that fixed devices stay in one location and measure surface currents as they flow past or through the device. These devices, which are often called current or flow meters, record water movements for scientists. Unlike floating devices, fixed devices are always intentional; that is, they are put in the water specifically to measure the surface currents.

## Indirect Methods

Alma understands how floating and fixed devices can be used to directly measure surface currents. But, what about indirect methods of measurement? There are three major types of indirect methods to measure surface currents.

The first involves measuring pressure gradients in the water. A pressure gradient is the direction that the water is being pulled. Measuring the way that the water is being pulled tells scientists how the surface currents are moving.

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